iio: adc: palmas_gpadc: retrieve a valid iio_dev in suspend/resume

commit d1aaf20ee655888c227d5137b7a63551f8d15416 upstream.

The suspend/resume functions were using dev_to_iio_dev() to get
the iio_dev. That only works on IIO dev's.  Use dev_get_drvdata()
for a platform device to get the correct iio_dev.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>